A New Filter for Life’s Survival

byPaul GilsteronJanuary 22, 2016

How do we make out the odds on our survival as a species? Philosopher Nick Bostrom (University of Oxford) ponders questions of human extinction in terms of a so-called Great Filter. It’s one that gives us a certain insight into the workings of the universe, in Bostrom’s view, because it seems to keep the galaxy from being positively filled with civilizations. Somewhere along the road between inert matter and transcendent intelligence would be a filter that screens out the vast majority of life-forms, keeping the population of the galaxy low, and offering us a way to gauge our own chances for survival.

Think of it this way. Perhaps the Great Filter has to do with the formation of life itself. If that is the case, then we have already made it through the filter and can go about exploring the universe. But if the Great Filter is in our future, then we can’t know exactly what it will be, and neither can we know whether we will survive it. Here the final term in Frank Drake’s equation comes to mind, the one specifying the lifespan of technological civilizations. Perhaps the Great Filter has to do with technology destroying itself. In that case, the filter could spell our doom. For more on Bostrom’s ideas, see Bostrom: From Extinction to Transcendence.

Through the Gaian Bottleneck

An ominous view, some would say, but along come Aditya Chopra and Charles Lineweaver (Australian National University), with a new paper suggesting a different kind of filter. The authors call it the Gaian Bottleneck, and it’s a filter that life on Earth has already passed through. The scenario is that life is fragile enough that it rarely develops into intelligence.

The reason: Young planetary environments are unstable. The life that does emerge needs to find ways to regulate greenhouse gases like water and carbon to keep surface temperatures in the habitable range. Normally we think of the decrease in the incoming bombardment of Solar System debris going back 4.5 billion years and extending to about 3.8 Gya as being a key to making the Earth more suitable for life, but the Gaian Bottleneck sees early life as being under strong selection pressure to modify and regulate its own environment. From the paper:

… bombardment rates inevitably decrease in the circumstellar habitable zones (CHZs) of stars, but the timescales for the evolution of Gaian regulation are probably unpredictable and would not inevitably evolve rapidly (or at all). Thus, if there is anything special about what happened on Earth to allow life to persist here, it might have less to do with the decreasing bombardment rate in the Hadean, or special chemical ingredients, or sources of free energy, or even a rare recipe for the emergence of life. The existence of life on Earth today might have more to do with the unusually rapid biological evolution of effective niche construction and Gaian regulation in the first billion years. Habitability and habitable zones would then not only be a passive abiotic property of stellar and planetary physics and chemistry (such as stellar luminosity, initial water content, and decreasing bombardment rate) but would also be a result of early life’s ability to influence initially abiotic geochemical cycles and turn them into the life-mediated biogeochemical cycles that we are familiar with on the current Earth…

In this view, we have gotten through the filter already, finding ourselves in a position not shared by planets around us. Conceivably, both Mars and Venus were once habitable, but a billion years after formation, Venus turns into the hell it is today and Mars goes into a deep freeze. Chopra and Lineweaver argue that if there was early microbial life on either world, it was unable to stabilize its environment, whereas on Earth, life played an active role in doing just that.

Image: Early Abiotic Feedbacks. During the first billion years after the formation of Earth (or of Earth-like planets), abiotic positive feedbacks (left) can lead to runaway surface temperatures outside the habitable range (both too hot and too cold). These positive feedbacks lead to the loss of liquid water [either from hydrogen escape to space or condensation into ice. Abiotic negative feedbacks (right) have been invoked to stabilize surface temperatures, but they may not be significant in the first billion years, hence the dashed lines and the question marks. As life evolves, it can strengthen or weaken these initially abiotic geochemical feedback loops and turn them into biogeochemical cycles and feedback loops. Evolving life can insert itself into these feedbacks at the points labeled A, B, C, and D. Credit: Aditya Chopra and Charles Lineweaver/ANU.

We can thus look without surprise on a galaxy where rocky terrestrial-class worlds are common even while life itself is not. Early extinction becomes a near universal phenomenon:

We argue that the habitable surface environments of rocky planets usually become uninhabitable due to abiotic runaway positive feedback mechanisms involving surface temperature, albedo, and the loss of atmospheric volatiles. Because of the strength, rapidity, and universality of abiotic positive feedbacks in the atmospheres of rocky planets in traditional CHZs, biotic negative feedback or Gaian regulation may be necessary to maintain habitability.

What, then, do we find on the planets of nearby stars? Not ancient ruins of advanced civilizations that have long ago destroyed themselves, but the fossilized remnants of extinct microbial life. No shattered remains of cities but long-gone remnants of simple life that failed in its mission to make its own planet a place where more complicated forms could develop. We would be living in a universe where almost all life is young, microbial, or extinct.

Chopra and Lineweaver run through a variety of ways their argument could be challenged. If Gaian regulation, for example, is a key to making a young planet into a place where life can survive, why should it necessarily be rare? Moreover, Gaian regulation is usually considered as starting in the Proterozoic, around 2.5 Gya. Invoking a pre-Proterozoic Gaia is even more controversial, to say the least. And the paper notes that abiotic negative feedback on the carbonate-silicate cycle could have stabilized surface temperatures on Earth without any recourse to Gaian regulation. Other potential objections are also weighed.

But knowing how many imponderables they are dealing with, the authors nonetheless suggest this sequence of events, which they consider ‘potentially universal,’ on young, wet planets:

First *0.5 Gyr: Hot, high bombardment, uninhabitable.*0.5 to *1.0 Gyr: Cooler, reduced bombardment, continuous volatile loss.*0.5 to *1.0 Gyr: Emergence of life in an environment with a tendency to evolve away from habitability.*1.0 to *1.5 Gyr: Inability to maintain habitability, followed by extinction. As a rare alternative, this period would experience the rapid evolution of Gaian regulation and the maintenance of habitability, followed by the persistence of life for several billion more years.

The paper is Chopra and Lineweaver, “The Case for a Gaian Bottleneck: The Biology of Habitability,” Astrobiology 16(1) (January 2016), 7-22 (abstract).

I am a little dubious about the whole concept of “gaian regulation.” It seems more likely to me that early life is often self-destructive. Consider Earth’s own history: the evolution of photosynthesis sucked all the carbon dioxide out of the atmosphere, kicking off several hundred million years of “snowball Earth” conditions which life barely survived. Only the gradual increase in the Sun’s output ended that period. Had it happened at a different time in the Sun’s life cycle, the world might have remained glacial. Or if photosynthesis didn’t develop, the increased heat and a very greenhousy atmosphere would have made Earth a twin of Venus.

The idea that life regulates its environment is very dodgy and seems to verge on faith rather than science.

‘The idea that life regulates its environment is very dodgy and seems to verge on faith rather than science.’

I agree with you Cambias, the biosphere has more to do with energy budgets, competition of organisms, resources and the average effect of these processes than a spiritual concept of “gaian regulation”. But each to their own.

Think about evolution. Which organisms will be fitter, those whose existence stabilizes their environment, or those that don’t? Given eons, it’s conceivable that organisms that help stabilize the environment at least to a point of sustaining life will become prevalent.

The snowball Earth was ultimately believed to have ended through abiotic processes rather than the sun. Undersea volcanic eruptions maintained thaw holes through the ice and ultimately returned the atmospheric CO2 to their former higher levels. The temperature rose through the resultant greenhouse effect which was then stopped from becoming “runaway” through either biotic negative feedback and more controlled photosynthesis or a return of abiotic silicate weathering with no ice sheet to prevent it . Or perhaps a bit of both .

This makes a lot of sense. I see three conditions that need to be satisfied to get complex life on a planet:

1) conditions have to be right for abiogenesis at some point in time
2) abiogenesis has to actually happen during that time
3) conditions have to stay right a sufficiently long time for evolution to do its work.

The present work addresses 3), and correctly notes that Gaian regulation can help out with it, greatly. The question of which of the three is the bottleneck, i.e. the actual Great Filter, is unresolved. My favorite is 2).

Only, I don’t think it solves Fermi because we haven’t yet got a clue the probability of a planet with life becoming a stable biosphere.

And I don’t buy the “great filter” because there is no compelling evidence of absence that is not based on simplistic expectations of how an”advanced” society would surely behave. Maybe unchecked growth and wasteful energy expenditure are incompatible properties for civilizations with longevity?

This is an interesting idea. I’ve always felt that the idea that the Great Filter is in our future is implausible. After all, even on Earth, we have a great variety of cultures. It’s hard to imagine that all the diverse cultures of the entire universe are making the same mistakes that spell their doom.

I agree with you, karlo. The Great Filter is in our past, not in our future. Either something involving early earth environment, as Chopra and Lineweaver suggest, or something involving the incredibly unique and amazingly lucky freak accidents that led to the evolution of the human race (including the final shifts in climate in the Rift Valley that probably got us up on 2 legs).

One side note that I don’t see mentioned in any discussion of Chopra and Lineweaver hypothesis: Mars is barren because the atmosphere has been blown away by solar wind, because Mars lacks a strong enough magnetic field to block that action of the solar wind. If Mars had a molten core, like Earth, and a strong magnetic field, like Earth, then maybe we would see a Green Mars right now, not a Red Mars. Of course, lack of magnetic field can’t be the Great Filter. Of all the rocky planets in the galaxy, a lot of them probably have molten cores and strong magnetic fields. I only mention this because our nearest neighbor and saddest would-be-candidate for life, fails on this score.

The implication of this bottleneck thesis by N. Bostrom is that
living organisms must integrate themselves into the geophysical
evolution of a planet to survive . If so then as it is implied is a jumping off point that few biota have attained in the galaxy.
One thing to ponder, on the Earth CO2 is regulated by biomass consumption and volcanic out gassing. The recycling of CO2 and other critical chemicals only takes place because of Plate tectonics. Without it is probable that a resurfacing event such as Venus’ would occur on most terrestrials. Would not that kill most microorganisms whether on the surface or in the upper crust.?
Can planetary astronomers think of any other mechanism than a Theia
event that emulates plate tectonics, without a resurfacing event?

Great post, Paul! I cannot help but imagine combining the Gaian bottleneck scenario with one where abiogenesis is extremely unlikely to occur to begin with due chemistry-related factors. At that rate, we would be looking out into a very lonely galaxy with countless sterile worlds in the HZ of their stars. The nearest life, let alone intelligent life, might be in the Andromeda galaxy or even further away. These two scenarios, when combined, would basically come close to solving the Fermi paradox, right?

This is so true! The most emotional moment in a local natural history museum I’ve ever had was a piece of rock with something like banded iron formations [1], evidence of the great oxygenation event [2]. Touching that, I thought ‘This is the first big mark that life left on the the planet’. 2.3 billion years ago, life won!

I love that. To be fair, my own enthusiasm is influenced by a severe addiction to oxygen. (Don’t judge — it runs in the family.) But sitting at this end of the cosmic calendar, it stuns the mind to appreciate a billion years of hard work by our terraforming ancestors. Worlds aren’t easily made.

Dr. Behrhoozi and Dr. Peeples suggest that this much-calmer universe will go on to form over 10 times the planets that currently exist, with 92% of its Venus-Earth-Marslike planets yet to come. Fermi’s Galactic House Party might still happen, Earth is just unfashionably early.

In a sense I find this rather unfalsifiable. If abiogenesis happens, then either life continues or it doesn’t. Gaian feed backs help, but we really don’t know how important this is.

This hypothesis also seems to ignore possible panspermia mechanisms that allow life to establish at any point in time of a planet’s history. Possibly even more important is directed panspermia, with ETI deliberately seeding worlds with life, something we may attempt in the future with dead worlds.

Unfortunately speculation like this has no data to grind on. We can hope that within a few decades we will have evidence of living worlds ( or their absence despite favorable conditions).

The one data point we have, Earth, suggests that civilizations take 1/3 of the age of the galaxy to emerge. No other Earth species achieved this, so civilization might be very rare.

In which case the “great filter” is behind us. The absence of other civs is due to a low probability of emergence. (Or more ominously that life has been erased by agency).

“No other Earth species achieved this, therefore it is unlikely” is false logic. The endpoint here is not “Homo sapiens evolves civilization”, but rather “Any species evolves civilization”. This latter endpoint is made more plausible, not less, by the presence of many not-quite civilized species.

Great post, and a really interesting idea! I’m not sure why there has to be a single, “Great”, filter though. I suspect that the formation of simple (prokaryotic) life, its ability to stabilize early unstable worlds (Gaia), the evolution of more complex (eukaryotic) life that can extract more energy from its surroundings, the evolution of multicellular life that can form nervous systems, and the evolution of intelligence that can build tools and store information from generation to generation are probably ALL great filters.

I’m in the optimistic camp and suspect we’ve gone through the greatest filters, and will become an interplanetary species before some island of humanity (eg Earth, Mars) faces an extinction level event.

I think there is still too little data to make such assumptions.
Until we get atmospheric and eventually visual data of other Earth-like planets in other solar systems we can’t really claim we know much.
Just remember how flawed our assumptions about Pluto for example were before we have seen the world up close.

As to this theory, it sounded very depressing and pessimistic to me first.
But then I realized it would put humanity into a role of shepherds and curators in the universe, with our eventual destiny to seek newly born worlds and life and nurse it until it could evolve on its own. Perhaps post-humanity would do so and wait like Frederick Pohl’s aliens in black holes, slowed in time until future life eventually would gain sentience and be capable of contact? Remember that the vast majority of Earth like worlds are still waiting to be born(apparently 92%)http://www.sciencedaily.com/releases/2015/10/151020104849.htm
That is if the theory would be proven to be true, of which I am skeptical.
I remain convinced that we still have very little data on which to base such assumptions.

The central premiss of the “Gaian feedback” paper is based on the failure of supposed biotic negative feedback to stabilise the early environments of Venus and Mars into those conducive to life and a greater emphasis on the role of life’s contribution to biotic feedback loops . It plays down the role of negative abiotic feedback whilst bigging up positive abiotic feedback. Surely an alternative view is that the abiotic feedback loops were totally governed by Venus being too near and thus too hot and Mars too far ( small) and too cold . I once saw a well described argument as to Venus and Mars being far more habitable simply by swapping places ( Jumping Jupiters not withstanding ) . It has been argued for some time and well ( on the premiss of false positive O2 ” bio signature ” findings ) that abiotic processes could produce habitable planets in the absence of life.
“Habitable” not withstanding these theoretical planets have stabilised their environments over extended periods via abiotic processes. Kopparapu’s “Dune ” desert planets for instance, with the key factor here being low initial volatiles and especially water ,acting as an additional ( and potent ) abiotic negative feedback loop on Planets at the inner edge of a less conservative habitable zone.

“Jumping Jupiters not withstanding” – what a prescient expression, I haven’t heard for many years.
“If Gaian regulation, for example, is a key to making a young planet into a place where life can survive, why should it necessarily be rare?”
I think that is a key question. Why should we suppose that Earth’s evolution of the biotic component of negative feedback loops is particularly rapid (as opposed to other life, but not necessarily as we know it)?
Evolutionary success implies adaptation to a stable niche — whatever niches are stable in a given environment. Consider how the shark, the crocodile, the cockroach have prevailed. There is logic in supposing that the evolutionary principle of survival leads to the emergence of a sustainable biosphere (the sum of all niches). At least one sustainable between the end conditions of a habitable zone.

We are still looking at it from our point of view, super earth’s should have a huge biomass and could be well protected from outside influences. The Earth maybe on the edge as far as intelligent life is concerned and the most common type of civilizations may be similar to octopuses!

When I think of the Fermi Paradox I think of a primitive tribe on a remote island. They wonder about other islands and other people out there in the ocean. They set up observation posts to detect distant drum beats and smoke signals from afar. Their SETI (Search for Extra Territorial Intelligence) program has been unsuccessful so far, and now they are thinking that perhaps there is nobody out there.

But the ocean around them is full of boats that they don’t hear, the sky above them is full of airplanes that they don’t see, and advanced Extra Territorial civilizations broadcast radio and TV all the time by using electromagnetic waves that the island people haven’t discovered yet.

It’s too soon to think of Great Filters. “There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy,” rightly wrote William Shakespeare in Hamlet. I guess advanced galactic civilizations have found more things than we are presently able to imagine. I guess they use communication technologies based on physics that we haven’t unveiled yet, and travel among the stars as uploads carried by radiation beams.

The idea of a Great Filter has more to do with our limited imagination than with the universe out there. Nick Bostrom has written about super-intelligent AIs, mind uploading, and post-biological evolution. Paul Davies, Martin Rees, and many other experts, think advanced galactic civilizations are post-biological and evolved much beyond our limited imagination.

I enjoy reading space operas and vintage science fiction, but I find it naive compared to, for example, Greg Egan’s galactic civilizations of software uploads with ultra-advanced technology but a small footprint on the physical universe. That is, I think, the Great Filter, but what it filters is our imagination.

But recall Fermi’s original thinking: there’s no evidence of ETI on Earth, and there should be, because it only takes a few million years to colonize the entire galaxy, even at slow speeds. The island dwellers would see jets and ships, no matter how remote and “primitive” their tribe.

My feeling is that the absence of ETI signals is real, not a failure of imagination or technology on our part. But I’d be very happy to be proven wrong, and I hope to live long enough to see SETI bear fruit.

“But recall Fermi’s original thinking: there’s no evidence of ETI on Earth, and there should be, because it only takes a few million years to colonize the entire galaxy, even at slow speeds.”

Sending tons of von Neumann machine into somebody home star (The Culture, MorningLightMountain Prime etc…) will guarantee interstellar war from multiple fronts. The idea works if and only if there is only one single civilization in the entire galaxy. Otherwise, the sender will receive several dozen relativistic rockets back at the home star or something even nastier showing up.

In fairness a Stone Age tribe would be much closer to us in time than a possible alien civilization. There is always a possibility that we observe effects of alien civilizations that we take for natural events or objects.

The idea of Von Neumann machines that go and colonize everything in the galaxy seems anthropomorphic to me, a reflection of the colonial past of many contemporary human cultures. Sure, a civilization advanced enough could do that, but is that what they want to do? I guess they have better things to do.

Suppose we develop the technology to send self-replicating Von Neumann nanomachines to the stars. Wouldn’t we program them to shut down (or limit their growth) if they find life? I think we would.

True enough, but we have no reason to believe some aliens WON’T have customs resembling our “colonial past”. We don’t know.
The assumptions spawned at the beginning of the space age, and any assumptions based on self-aggrandizement and self-hatred, are simply shots in the dark. I admit a personal bias against blaming ourselves for everything wrong but I stand by it.

I believe there are places in the southern hemisphere, especially the Indian Ocean, where there is no shipping routes or aircraft flight routes that primitive islanders could see them. These would be ideal areas to put up observation post for UFO’s.

Giulio, an excellent post. We often talk about Dyson Spheres, Matrioshka brains and Stellar Engines but these are concepts and things that we can imagine already as a civilization. We can’t really say what will be a challenge or prospect for us in 5000 years, or for a civilization one million years ahead.
Still, we have to go by what we know and can visualize as possible now.

Thanks. We can see examples now. In the 60s we thought of flying cars. In the 10s we have the technology to build flying cars (I guess personal helicopters could be produced at reasonable cost if the market demanded it), but nobody thinks flying cars make much sense. Napoleon would have seen uses for airplanes, but not for the Internet (actually I guess he would have been horrified by the idea of strong young men glued at a screen all day). Who can predict the values and priorities of our grand-grandchildren?

But the ocean around them is full of boats that they don’t hear, the sky above them is full of airplanes that they don’t see, and advanced Extra Territorial civilizations broadcast radio and TV all the time by using electromagnetic waves that the island people haven’t discovered yet.

Sure they will miss the radio waves, but why would they not hear the boats and see the airplanes? Are they deaf and blind? Certainly they would notice when the first visitors come to their island to say hello, build a resort, drill for oil, or set up a nuclear test site?

Something lost in this discussion is that the Fermi Paradox is just a theory. In truth, we don’t have any idea how prevalent life is in the Milky Way, intelligent or not. We don’t know that there are any filters at all, except for those we have evidence of, and if those filters didnt stop life here (and quite likely accelerated the rate of evolution by favoring more adaptable species), then what evidence is ther that life, or intelligent life, is rare? Advanced technical sivilizations that don’t direct beam energy to our planet and which don’t visit or build Dyson structures have not yet been detectable. The galaxy could be packed with intelligent life, for all we know.

Also, life stabilizing a planet’s environment doesnt’t seem so unlikely, if abiogenesis occurs. How uncommon is the arise of coexisting lifeforms like plants and animals that each use the gaseous output of the other? Wouldnt a planet with a mix of such life always create a degree of stability in its atmosphere?

Exactly. We simply don’t know enough even about our own area, as the possibility of yet another outer planet shows.
There’s nothing wrong with speculating and revising speculations but our priority should be to explore our neighborhood and occupy usable places while developing space biospheres.
In fact,we should avoid getting the attention of any aliens that may exist till we are firmly entrenched — if that word is suitable — in space. Nothing wrong with listening for them, though.

‘In fact,we should avoid getting the attention of any aliens that may exist till we are firmly entrenched — if that word is suitable — in space. Nothing wrong with listening for them, though.’

I believe this too, we should listen out for them rather than waste a lot of energy trying to communicate. If we see them and they are localised in space a fair distance away so it takes them a long time to get here then maybe as it would give us plenty of time to prepare. You just never know, I don’t fancy been a zoo exhibit.

I really think the Great Filter is located at the transition between bacterial-grade and eukaryote-grade life-forms. The differences in cellular architecture, sub-cellular organization, genomic structure, etc. are very profound. Unlike other evolutionary transitions there does not seem to be a probable mechanism. It bears the hallmarks of being a fluke.

Aspects of eukaryotic cells that make them morphologically flexible, such as the cytoskeleton and the mechanisms of gene expression, seem peculiarly “pre-adapted” for the transition to multicellular lifeforms with differentiated tissues. Since evolution does not exhibit foresight, I take this as indicative of a low probability event.

I can understand the strong urge to understand why we may or may not be alone in the universe. I just find it difficult to see the point of these sorts of extremely speculative ruminations. To date we have only one single biosphere with one single known ‘intelligent’ civilization (with one particular line of historical progression ) on which to base anything. Much of it also just seems to reflect social concerns of a particular era in such as the idea that technological civilizations wind up ‘destroying themselves’. This seems to be overly anthropocentric – how do we know another civilization would even develop technologically in the same way? How do we know ‘intelligence’ is reflected the same way in a different solar system?
Simple answer – we don’t.
The latest of these I heard in the past couple of weeks blames ‘climate change’ as a likely culprit in preventing advanced life. Really? Sounds to me more like psychological projection than science.

Exactly. Projection, particularly, of whatever emotions are the fad — self-hatred in the Western world, apparently because we defended ourselves in World War II and faced down subsequent enemies. That’s as over and gone as the legal racial segregation too many people are still trying to struggle against.

Right now our speculations are similar to those of Classical Era thinkers like Ptolemy (the scientist, not to be confused with the rulers by that name). Based on all he knew he at least got the shape of the Earth correct as a sphere but he thought it was central to a series of crystal spheres with the various planets and stars on them that enclosed and moved around the Earth in specific patterns.
This is more or less what we’re doing as long as we don’t have other biosystems to study.
I’m not saying speculation isn’t a good or useful thing but right now whatever we come up with is almost certainly as inadequate as Ptolemy’s theory — which was misleading till Galileo.

Interesting, but highly speculative, and possibly too pessimistic: instead of speaking of Great Filters, each one leading to total extinction, I am more inclined to believe in Great Barriers, each one inhibiting further development.

In other words: I think that life on Earth has shown that once it gains a foothold, it is extremely tenacious and resilient, surviving Late Heavy Bombardments, Snowball Earth, O2 poisoning, etc.

At the same time it has also shown that it can take extremely long for the Next Big Step, if it ever occurs at all.
This also raises the issue of the so-called window of opportunity for complex life, which is much shorter than the main sequence lifespan of a star, see this post on CD:https://www.centauri-dreams.org/?p=25359&cpage=1#comments

All together, I tend to believe that life may be rather or even very common in the universe, in a primitive state (single-celled, Prokaryotic), but that higher (Eukaryotic) and complex (multi-celled) life may be very rare, and intelligence exceedingly rare.

With regard to the last issue, intelligence and technological civilization, risking to mention the obvious (there have been some studies in recent years with regard to the occurrence/absence of K3 galaxy-wide civilizations), I have been reading several interesting recent articles lately, about our great neighbor, the Andromeda Galaxy (such as “Stellar mass map and dark matter distribution in M31”), and would like to mention the following facts;

– Andromeda contains about 800 – 1000 billion stars.
– Andromeda has a very large and long-term stable disc region.
– Most of Andromeda’s stars (by far) are over 7 gy old (it has undergone a great starburst about 7 – 8 gy ago, as a result of some mergers with smaller galaxies).

In other words, Andromeda has had plenty of time (2 – 3 gy ahead of us) and opportunity to produce an advanced (i.e. advanced K2 or K3) civilization.
But no signs of it whatsoever (signals, mega-artifacts, colonization of our MW galaxy, …) . Unless we don’t know what an advanced civilization does and looks like. But all this does not make me optimistic about the prevalence of intelligent life in the cosmos.

How true, I fully agree with you!
If indeed intelligent life and civilization are extremely rare, then the upside of this is no competition, no galactic pecking order, no evil aliens. And lots and lots of space for expansion.
And yes, I also agree that THE way to ensure our own survival is to become a multi-planetary system/interstellar species.
Personally I even see this as our responsibility and some sort of ‘galactic calling’ (the closest I would venture to ‘destiny’, be it a self-chosen one): to ensure the survival and expansion of our intelligence and civilization, and to seed lifeless, but (potentially) habitable planets with life.

I agree that life is like a pyramid, in which simple forms of life are very common, and intelligent life is rare. In other words, the complexity of life is inversely proportional to its commonality.

My opinion is that the great filter is sentience/high encephalization quotient. High EQ is necessary for abstract thought, language, and complex social relationships, which are necessary for civilization, which is necessary for life to ensure its survival and prosperity.

I would like to learn more about abiogenesis: how does the primordial soup create organisms? etc.

“In other words, the complexity of life is inversely proportional to its commonality.” I fully agree with that, and would like to note that the absence of life is the overwhelmingly least complex form of it, and therefore the most common.

Spaceman, this will one day probably be elevated to the level of Natural Law. Do not forget to publish this and attach your name to it for eternity :-)

“My opinion is that the great filter is sentience/high encephalization quotient”
Again, I fully agree!

“I would like to learn more about abiogenesis: how does the primordial soup create organisms? etc.”
Wouldn’t we all?! This is one of the ultimate questions.

All together, I think that what we need and will ultimately (hopefully) get, is a kind of refined Drake equation, where Fl, the fraction of planets that develops life, should be split into more factors, or more factors should be inserted between Fl and Fi (fraction of planets that develops intelligence).

@Ashley Baldwin: I liked your post, especially how you zeroed in on the central premise (or premiss) of the paper’s thesis! I had never heard of the idea that swapping Mars and Venus in their orbits would make each of them more habitable though it makes sense, as I have heard of the argument that Mars would have had a greater chance of being habitable if it was a larger planet. You mentioned “negative abiotic regulation” to make a planet habitable. I would love to learn more about abiotic regulation to make a planet habitable…do you remember who wrote the paper on false positive bio-signatures as a result of abiotic regulation of O2? If there is an emergence bottleneck, then negative abiotic regulation would certainly be a good friend to would-be interstellar colonists—no native life to contend with AND a habitable planet all in one! I cannot help but wonder if the authors would have gotten less flak if they substituted another term for “Gaian”?

Speaking of flak, I have to say that even though I know emotions run high on this topic, it is disrespectful and uncivil to refer to this paper as “rubbish.” Calling the work pessimistic is one thing, but using insulting terms is another. Dr. Charley Lineweaver is a well-educated, highly accomplished scientist. He and his colleague are working with limited data and, as I see it, trying to come up with another explanation for the apparent lack of life in our Universe. As Ronald’s excellent example of the Andromeda galaxy illustrates, it is very puzzling that we continue to see no evidence of technological presence in the Universe (even though some argue that we still have a significant percentage of the parameter space to sift through). In the spirit of true critical thinking, the Chopra and Lineweaver identify a number of objections and limitations to their hypothesis. Also, even if there are cases of negative abiotic feedback at work in other solar systems, it is certainly possible that, in the vast majority of other cases, biotic negative feedback will be needed to sustain the habitability of a wet rocky planet. Where is the bottleneck? Why is it that we see no evidence of life within our solar system and no evidence of super-civilizations in deep space? These are some of the most fascinating questions science. As I mentioned in an earlier post, an emergence bottleneck combined with, dare I say it, a Gaian bottleneck would come close to solving the Fermi paradox. There are many proposed solutions to the FP and this one seems more reasonable than many I have come across.

Just how much the snowball Earth stalled the development of multicellular life my answer the question of advanced life forms. If we look a the earth as only being around for some 700 million years in the evolution sense it may indicate how common multicellular life is and how even an A class star could have civilizations. The development of the eye is probably going to be the road to advanced intelligents as we see in octopuses, cuttlefish, squid and spiders. The ability of advanced forms to develop on large Mars to mini Neptunes and even internal oceans on smaller worlds, I would expect intelligents to develop in the large oceans of these worlds. After all 65 million years ago we were just puny little mice! The development of aquatic nonmammal intelligents and civilizations is still look upon as fantasy and science fiction.

So where are all the cephalopods? Maybe they are segregationist, something that is common to many advanced species, plus our own problems caused by the ancient instinct to dominate a territory, just look at the world map. The idea that we put our own fears to relate how other intelligents might develop is not to far off but we need to look back at our primitive instincts to see what the root causes may be for other civilizations and that may be competition and challenges.

Mike,
While it is true we only have “one single biosphere”, it is not true that we have to base our speculations on it. We have a mountain of science and engineering knowledge, astronomy to QED, which provides bounds and scales for the speculations. Your criticism of the Great Filter of the Week is valid, but that does not mean well-based speculation is not possible or useful.
Check my blog in the bloglist here.

Actually from the viewpoint of m0deling other exoplanets we have two biosphere: the salt water oceans and the land. We do not consider the oceans on equal footing with us because we do not live in it, yet it covers 3/4 of the planet. We know more about the surface of the moon than we do about the seabed. This means that we are much more conscious of land animals and recognise them more readily. The long term stability of ocean worlds may make that the birthplace of most civilizations, just like us! That is why we need to understand how that they might communicate. The octopus is a good example and because it has such a short life cycle thou, we need to model how it could develop into a more advanced species.
How many Sci-Fi movies have tentacle aliens?

Numerous books have been penned on the exciting subject of life on other worlds, and the news almost daily carries stories about potential new habitable sites, ranging from water on Mars to oceans on Europa and exoplanets beyond the solar system.

The Impact of Discovering Life Beyond Earth picks up where those books leave off and asks “what would happen if we actually discovered life?” The answer depends, of course, on the discovery scenario. The reaction to the discovery of microbes would be quite different to the reaction to the discovery of intelligent life. And if a message is deciphered, the reaction would quite different again, depending on the content of the message! All these scenarios and many more are covered in this highly original volume, written by scientists, philosophers, historians, anthropologists and theologians addressing the new field of astrobiology and society.

All I will say at this point is that aliens will be alien. That is why they are aliens. Then again they could be a lot like us with a few funny looking appendages.

Though since no one is asking, I think the first ETI we will find (and I do not necessarily mean make contact with) will be some version of Artilects – which may in turn explain why that type has not tried to contact us.

Any potential risks of METI are negligible relative to those simply posed by the discipline of listening, watching and learning, perhaps even of quieting our own broadcast noise while we do it? Sure, because it’s “to our best advantage if they know us, if they know we’re here already, to take the message into our hands and portray ourselves as we wish rather than accidentally portraying ourselves [in a bad light].”

While that enthusiasm is appealing, it’s hasty reasoning for such a soberminded crowd. Appropriate caution is a measure of what (or who) is potentially at stake. Perhaps most ethically troubling, ‘us’ here does not actually refer to us. It refers to future human generations and every species that will call Earth home for tens and hundreds of thousands of years to come. That’s who METI proposes to act on behalf of. No matter how remote the risk, no matter how well we believe we’ve figured out the game, I cannot think of higher stakes to place on the table.

Fortunately, nothing horrible has ever happened for reasons that no one anticipated.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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